1. Field of the Invention
The present invention relates to an art of obtaining information for a medical diagnosis on the basis of a magnetic resonance signal emitted from a subject.
2. Description of the Related Art
In perfusion imaging which uses an intravascular contrast agent, such as dynamic susceptibility contrast-magnetic resonance imaging (DSC-MRI) or dynamic contrast-enhanced MRI (DCE-MRI), blood vessels disturb a diagnosis and are therefore desirably excluded on a analytical map. Blood vessels can be excluded by image processing such as threshold processing from an image in which the blood vessels are already visualized. This, however, not only requires extra processing but also makes it difficult to set an optimum threshold. Moreover, an image of this kind is generally obtained with low spatial resolution to attach importance to a signal-noise ratio and time resolution. Thus, parenchymal portions located in the vicinity of the blood vessels are also excluded by a partial volume effect. In a method such as a deconvolution method which uses an arterial input function (AIF), blood vessel portions are used for an analysis and therefore cannot be excluded. A spin echo (SE) based method is said to have a higher blood vessel suppression effect than a gradient echo (GRE) based method, but the effect is insufficient.
In functional MRI (fMRI) which uses GRE based echo planar imaging (EPI), changes in the activity of tissue (tissue including capillary vessels) are observed. However, an in-flow effect and a blood oxygenation level dependent (BOLD) effect in a relatively thick blood vessel hinder to the observation of changes in the activity of tissue adjacent to blood vessels in particular. In general spin-warp type GRE which does not use the EPI method, the in-flow effect, in particular, is an artifact for the original BOLD. The image processing has the same problem. In addition, recently reported diffusion-weighted (DW)—fMRI (see “Le Bihan at al. PNAS 103, 8263-8268, 2006”) uses a b-factor of 1000 or more, and is mainly directed to separate the variation of a D-coefficient in tissue cells in accordance with the difference of D-coefficient in a tissue interstitial fluid and an intracellular fluid.
Thus, the existing problem is that if fluids flowing at a lower velocity, such as the blood and contrast agent flowing in capillary vessels are visualized, fluids flowing at a higher velocity, such as the blood and contrast agent flowing in arteries and veins, are also visualized.